1991 Mercedes Benz 300e Car will not Move forward, but will

What is the general cost of replacing a transmission for this car. I have been quoted $4000 which seems high.
Additionally, I believe the blue book is around $2500 so I'm having trouble rationalizing spending for $4000 for a 19 year old "great condition but not running" car that if total would not recoup my repair investment. Your thoughts please.